1. Field of the Invention
The present invention relates to an optical system for an optical information recording/reproduction apparatus for recording information on an optical disk as an optical information recording medium, or an optomagnetic recording medium by focusing light via an objective lens, and/or reproducing information on the basis of light reflected by the recording medium.
2. Related Background Art
In order to optically or optomagnetically record and/or reproduce information, a conventional optical information recording/reproduction apparatus employs the following optical system. More specifically, this optical system will be described using an X-Y-Z three-dimensional coordinate system, as shown in FIG. 1. Divergent light components (beam) 171 from a semiconductor laser 101 are collimated into parallel light components 172 by a collimator lens 102, and the parallel light components (beam) 172 propagate in the positive direction of the X axis. Some of the light components 172 are reflected by a polarization beam splitter 103, become light components (beam) 173 propagating in the negative direction of the Z axis, and reach an APC sensor 104. The sensor 104 detects the amount of the light components 173, thereby detecting the emission amount of the semiconductor laser 101. For example, when the emission amount of the laser 101 is drifted from a desired emission amount under the influence of a change in environmental temperature, the emission amount of the semiconductor laser 101 is controlled to have a desired value using an APC control circuit (not shown).
On the other hand, light components (beam) 174 transmitted through the beam splitter 103 sequentially change their propagation directions via a galvano mirror 105 and a mirror 106, and are then focused as a light spot on a recording surface of a recording medium 109 such as an optomagnetic disk by an objective lens 107. The mirror 106 and the objective lens 107 constitute a movable optical system 108 as a unit (portion surrounded by a broken line), and can be moved in the radial direction (indicated by an arrow 151) of the recording medium 109. That is, the mirror 106 and the lens 107 constitute a so-called separated optical system, and allow a high-speed access operation.
Light components (beam) 175 reflected by the recording medium 109 propagate along the above-mentioned optical path in the reverse direction. Some of the light components 175 are reflected by the beam splitter 103 as light components (beam) 176 propagating toward the positive direction of the Z axis, i.e., toward a 1/2 wavelength plate 110. In this case, the divergent light components 171 from the semiconductor laser 101 are linearly polarized light oscillating within the X-Z plane. However, the 1/2 wavelength plate 110 is set so that the direction of polarization of transmitted light components 177 (beam) forms an angle of 45.degree. with respect to the X-Y plane. Therefore, the transmitted light components 177 are polarized and split by a polarization beam splitter 112 into light components (beam) 178 propagating toward a sensor 113 consisting of light-receiving units 113.sub.1, 113.sub.2, 113.sub.3, and 113.sub.4, and light components (beam) 179 propagating toward a sensor 114 after they are transmitted through a sensor lens system 111 including a spherical lens and a cylindrical lens.
In this prior art described above, auto-focus control (to be referred to as AF control hereinafter) is performed by an astigmatism method. More specifically, an AF control circuit (not shown) drives the objective lens 107 in the Y-axis direction on the basis of a differential signal between the sum of outputs from the light-receiving units 113.sub.1 and 113.sub.3, and the sum of outputs from the light-receiving units 113.sub.2 and 113.sub.4.
On the other hand, auto-tracking control (to be referred to as AT control hereinafter) is performed by a push-pull method. More specifically, an AT control circuit (not shown) rotates the galvano mirror 105 about a rotational axis 115 which is present in the X-Z plane, and forms an angle of 45.degree. with the X-axis (arrow 152) on the basis of a differential signal between the sum of outputs from the light-receiving units 113 and 113.sub.2, and the sum of outputs from the light-receiving units 113.sub.3 and 113.sub.4. Thus, the light spot is moved in the radial direction (arrow 153) on the recording medium 109.
In this manner, in the separated optical system, since the galvano mirror 105 is arranged on the stationary optical system side, the weight of the movable optical system can be reduced, and this is advantageous in terms of high-speed access.
Note that an optomagnetic signal is reproduced using a differential signal between the sum of outputs from the light-receiving units 113.sub.1 to 113.sub.4, and the output from the sensor 114.
However, when APC control is made using the output from the sensor 104 such as in the prior art, an unstable factor may be added to the control. More specifically, in order to perform precise APC control, the ratio of the light amount of light components 174 transmitted through the beam splitter 103 and propagating toward the objective lens 107 to the light amount of light components 174 propagating toward the sensor 104 must be constant. The transmittance, reflectance, and absorption coefficient of a film of the beam splitter 103 may slightly drift over time or under the influence of the environmental temperature or humidity. As a result, the above-mentioned light amount ratio drifts.
When the APC control is performed in such an undesirable state, since the semiconductor laser 101 emits light of an amount beyond a predetermined value, its service life may be undesirably shortened, and information recorded on the recording medium 109 may be destroyed in an information reproduction mode. When the semiconductor laser 101 emits light of an amount below the predetermined value in an information recording mode, an energy level necessary for recording cannot be assured on the recording medium 109, and a sufficient C/N ratio may not often be obtained in the reproduction mode. In addition, since the amount of light reaching the sensors 113 and 114 is small in the information reproduction mode, a sufficient C/N ratio may not often be obtained.
When the galvano mirror 105 is pivoted, the following problem is posed. More specifically, when the galvano mirror 105 is pivoted in a direction of the arrow 152 to perform AT control, light components are displaced in the X direction on the sensor 113, and the AT control signal is undesirably offset.
Thus, the pivotal angle of the galvano mirror 105 must be limited to a small angle, thereby substantially preventing a reproduction signal from being degraded. Instead, the movable optical system 108 must be precisely moved at high speed in a direction of the arrow 151. Alternatively, the pivotal angle of the galvano mirror 105 may be detected to obtain an offset amount based on the detection amount, and an AT control signal obtained from the sensor 113 may be corrected using the offset amount. FIG. 2 exemplifies a means for detecting the pivotal angle of the galvano mirror 105. In this means, the galvano mirror 105 is rotated about the rotational axis 152. For this reason, incident light components 174 are deflected from one state 181 to another state 182. The pivotal angle detection means is arranged on the rear surface side of the galvano mirror 105. Light components from an LED 121 are reflected by the rear surface of the galvano mirror 105, and the propagation direction of reflected light components is changed upon pivotal movement of the galvano mirror 105. Therefore, a sensor 122 having two light-receiving units 122.sub.1 and 122.sub.2 is placed in the optical path of the reflected light components, and paying attention to a difference 123 between outputs from the two light-receiving units, the difference 123 is almost linearly changed upon rotation of the galvano mirror 105. When the pivotal angle detection means is arranged in this manner, the offset amount of an AT control output can be corrected. However, a special means for detecting the pivotal angle must be prepared. As a result, the overall apparatus becomes complicated and bulky, and its cost is inevitably increased.